Power roller bearing of toroidal-type continuously variable transmission unit

ABSTRACT

A power roller bearing of a toroidal-type continuously variable transmission unit that is used for a transmission of an automobile or the like comprises a power roller, an outer race, balls, a retainer, etc. Annular raceway grooves are formed individually on the respective opposite end faces of the outer race and the power roller. Treated-surface layers for enhancing the endurance of the power roller bearing are formed individually on the respective surfaces of the raceway grooves. The treated-surface layers include super-finished surfaces with the surface roughness of 0.05 Ra or less, formed individually on the raceway grooves, and low-friction layers formed individually on the super-finished surfaces. Alternatively, the treated-surface layers include residual compression stress layers formed on the respective surface layer portions of the raceway grooves by shot-peening.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is a Continuation Application of PCT Application No.PCT/JP00/02201, filed Apr. 5, 2000, which was not published under PCTArticle 21(2) in English.

[0002] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Applications No. 11-098841, filed Apr. 6,1999; and No. 11-098842, filed Apr. 6, 1999, the entire contents of bothof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention relates to a power roller bearing of atoroidal-type continuously variable transmission unit adapted to be usedfor a transmission of, for example, an automobile, general industrialmachine, transportation equipment, etc.

[0005] 2. Description of the Related Art

[0006] A toroidal-type continuously variable transmission unit adaptedto be used for a transmissions of, for example, an automobile, generalindustrial machine, transportation equipment, etc. comprises an inputdisc and an output disc opposed to each other, a power roller providedbetween the input and output discs, a loading cam mechanism for pushingthe input and output discs toward each other, etc.

[0007] The input disc can be rotated by means of a drive source such asan engine. Power that is based on the rotation of the input disc istransmitted to the output disc through the power roller. The powerroller is provided for swinging motion between the input disc and theoutput disc. The power roller is provided with traction portions thatare in rolling contact with the two discs, individually. The powerroller is rotatably supported by means of a power roller bearing thatfunctions as a thrust bearing.

[0008] The power roller bearing is provided with an end portion of thepower roller that functions as an inner race, an outer race, ballsprovided for rolling motion between the outer race and the power roller,etc. The power roller is supported for swinging motion between the inputdisc and the output disc by means of a trunnion. The gear ratio of thetoroidal-type continuously variable transmission unit can be changed bychanging the angle of inclination of the power roller.

[0009] The power roller bearing constructed in this manner hardlyapparently differs from a conventional thrust ball bearing that is usedto support a rotating shaft on which thrust load acts except for thearrangement of the power roller. Accordingly, a study has been made toproduce power roller bearings at low cost by using parts that aredesigned for existing thrust ball bearings.

[0010] Although a power roller bearing apparently resembles a thrustball bearing, however, its power roller functions quite differently fromthe inner race of the conventional thrust ball bearing. Morespecifically, the distribution of load that acts on the power rolleritself and the behavior of the balls in contact with the outer race andthe power roller considerably differ from those of the conventionalthrust ball bearing, so that improvement must inevitably be made inconsideration of those differences.

[0011] While the inner race of the conventional thrust ball bearing is amember that simply supports a shaft, for example, the power rollerfunctions as a power transmitting member for transmitting torque fromthe input disc to the output disc. This power roller is equivalent to atransmission gear in a gear-type multistage transmission. The powerroller of this type rotates at high speed under heavy forces of pressurefrom the input disc and the output disc, so that it generates intenseheat. The heat from the power roller serves to heat the balls and thelike. Thus, it is essential to use high-viscosity traction oil, which isdeveloped for the purpose of power transmission, as lubricating oil tobe supplied between the outer race and the power roller.

[0012] The traction portions of the power roller that touch the inputdisc and the output disc face each other at a circumferential distanceof 180° on the outer peripheral edge of the power roller. The heavyforces of pressure from the input disc and the output disc intensivelyact on the traction portions of the power roller as a resultant forcethat combines radial load and thrust load. Thus, a very high contactpressure develops in the traction portions of the power roller thattouch the input disc and the output disc.

[0013] A conventional bearing is used under a contact pressure of 2 to 3GPa (gigapascals) or less, for example. In the case of a power rollerbearing that is used in a toroidal-type continuously variabletransmission unit for a vehicle, on the other hand, the contact pressureranges from 2.5 to 3.5 GPa in a normal deceleration mode. In some cases,the contact pressure may even reach 4 GPa in a maximum decelerationmode.

[0014] Further, the heavy forces of pressure from the input disc and theoutput disc intensively act as a radial load in positions at a distanceof 180° from each other on the traction portions of the power roller.This radial load causes the power roller to undergo compressivedeformation in the radial direction. Since this compressive deformationcauses the power roller to warp, it is hardly possible to allow aplurality of balls between the power roller and the outer race uniformlyto share thrust load that acts on the power roller. Thrust load thatacts on these balls increases in positions at an angular distance of 90°from the positions of contact (traction portions of the power roller)between the power roller and the input and output discs. In consequence,pressures of contact between the individual balls and raceway groovesvary, so that some of the balls roll in the raceway grooves under veryhigh contact pressures.

[0015] In order to prevent high contact pressures from lowering the lifeperformance, it is essential specially to adjust the materials, surfacehardness, and surface roughness of the traction portions of the powerroller in contact with the input and output discs and the respectiveraceway grooves of the power roller and the outer race that are touchedby the balls.

[0016] In order to lengthen the life of the power roller bearing, basedon this background, the applicant hereof proposed a technique in whichballs are formed of medium-carbon steel or high-carbon steel and thehardness and strength of the ball surface are adjusted bycarburizing-nitriding treatment or quenching and tempering treatments(see Jpn. Pat. Appln. KOKAI Publication No. 7-208568).

[0017] The applicant hereof also proposed a technique in which an inputdisc, an output disc, and a power roller in contact with them aresubjected to ground finish after they are subjected to carburizingtreatment. Further, the applicant hereof proposed a technique in whichinput and output discs and a power roller are subjected to ground finishafter they are subjected to carburizing-nitriding treatment so that thehardness and effective case depth of the respective surfaces of thesemembers can be adjusted to appropriate values (2 mm to 4 mm) that standlocally high contact pressures (see Jpn. Pat. Appln. KOKAI PublicationNo. 7-71555).

[0018] Although the hardness, effective case depth, and surfaceroughness of the power roller and balls are rationalized by usingtraction oil for power roller bearings, use of appropriate materials forthe individual members, and surface treatments, as mentioned before, theendurance of the power roller bearing can be improved only limitedly byit alone.

[0019] In some cases, the molecular structure of the traction oil,synthetic oil, may be decomposed as the power roller is heated, inparticular. In these cases, the traction coefficient worsens, and thesafety factor for gloss slip also lowers. If the traction oil isdegenerated, moreover, it is hard to form oil films on the respectivesurfaces of the power roller, balls, etc. Accordingly, there is apossibility of the traction portions of the power roller and thesurfaces of the respective raceway grooves of the power roller and theouter race undergoing early flaking. These phenomena cause the lifeperformance of the power roller bearing to lower.

[0020] Since the power roller bearing is originally designed for powertransmission, it is important to minimize loss of the dynamic torque ofthe bearing itself, thereby improving the torque transmissibility.Despite the aforesaid improvement, however, a substantial dynamic torqueloss may be caused to lower the torque transmissibility, depending onthe dimensions of the balls and the respective raceway grooves of thepower roller and the outer race. If the hardness and effective casedepth of the respective surfaces of the power roller and the like areadjusted in the aforesaid manner, moreover, the edges of the racewaygrooves or the balls sometimes may be broken early, or the respectivecontact surfaces of the raceway grooves and the balls may be marred, sothat the life performance of the power roller bearing may be lowered.

BRIEF SUMMARY OF THE INVENTION

[0021] Accordingly, the object of the present invention is to provide apower roller bearing of a toroidal-type continuously variabletransmission unit, of which the life performance can be restrained fromlowering.

[0022] A power roller bearing of a toroidal-type continuously variabletransmission unit of the present invention is provided for swingingmotion between an input disc capable of being rotated by means of adrive source and an output disc opposed to the input disc, and comprisesan outer race, a power roller in rolling contact with the input disc andoutput disc, and balls provided for rolling motion between the outerrace and the power roller, the outer race and the power roller havingannular raceway grooves in which the balls roll individually, at leastone of the respective raceway grooves of the outer race and the powerroller being formed having treated-surface layers for enhancing theendurance thereof.

[0023] The treated-surface layers based on a first aspect of the presentinvention include super-finished surfaces with the surface roughness of0.05 Ra or less, formed on both the respective raceway grooves of theouter race and the power roller, and low-friction layers formed on atleast one of the super-finished surfaces.

[0024] According to this invention, heat generated in the power rollerbearing can be reduced by lessening friction that is produced as theballs roll in the respective raceway grooves of the outer race and thepower roller. Thus, the respective surfaces of the raceway grooves arenot liable to flaking, so that the life of the power roller bearing canbe lengthened. According to this invention, the dynamic torque of thepower roller bearing is lessened, and the general power transmissionefficiency of the toroidal-type continuously variable transmission unitis improved. If the transmission unit is applied to an automobile,therefore, the fuel consumption ratio can be improved at the same time.

[0025] Preferably, the low-friction layers are formed of lubricativesubstances selected from at least one of materials including gold,silver, lead, molybdenum disulfide (MoS₂), tungsten disulfide (WS₂), andfluoroplastics. Among these substances, some of the lubricativesubstances including gold, silver, lead, or fluoroplastic are graduallyscraped from the low-friction layers by means of frictional force thatis produced as the bearing rotates. As the particles of the scrapedlubricative substances are transferred to a fellow surface that istouched by the balls, thin films of the lubricative substances areformed on the fellow surface, whereby the bearing is lubricated. Sincelayers of molybdenum disulfide and tungsten disulfide as the lubricativesubstances have their own lubricative surfaces, these lubricativesurfaces wear little by little as they develop lubricating properties.The layer of molybdenum disulfide or tungsten disulfide is formed on thesurface of each ball or the surface of the raceway groove of the powerroller or the outer race by sputtering. According to the power rollerbearing using these low-friction layers, heat can be restrained moresecurely, so that the life of this bearing can be lengthened.

[0026] In order to achieve the above object, the treated-surface layersof the power roller bearing of the present invention based on a secondaspect include residual compression stress layers formed by shot-peeningon at least one of the respective raceway grooves of the outer race andthe power roller. These residual compression stress layers serve to easestress that is generated as the balls roll in the raceway grooves. Thus,the fatigue life of the power roller bearing can be improved.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0027]FIG. 1 is a longitudinal sectional view showing a part of asingle-cavity half-toroidal-type continuously variable transmission unitaccording to a first embodiment of the present invention;

[0028]FIG. 2 is a sectional view showing a power roller bearing of thesame embodiment;

[0029]FIG. 3A is a plan view of an outer race of the power rollerbearing of the same embodiment;

[0030]FIG. 3B is a sectional view of the outer race taken along lineIII-III of FIG. 3A;

[0031]FIG. 4A is a plan view of a plan view of a power roller of thepower roller bearing of the same embodiment;

[0032]FIG. 4B is a sectional view of the power roller taken along lineIV-IV of FIG. 4A;

[0033]FIG. 5 is a sectional view showing a ball of the power rollerbearing of the same embodiment;

[0034]FIG. 6 is a flowchart showing some of manufacturing processes foran outer race and a power roller according to a second embodiment of thepresent invention;

[0035]FIG. 7 is an enlarged sectional view schematically showing thesurface of a raceway groove of the outer race or the power rollerobtained in the manufacturing processes shown in FIG. 6;

[0036]FIG. 8 is a flowchart showing some of manufacturing processes foran outer race and a power roller according to a third embodiment of thepresent invention; and

[0037]FIG. 9 is an enlarged sectional view schematically showing thesurface of a raceway groove of the outer race or the power rollerobtained in the manufacturing processes shown in FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

[0038] A first embodiment of the present invention will now be describedwith reference to FIGS. 1 to 5.

[0039]FIG. 1 is a partial sectional view of an automotive transmission21 that uses a single-cavity half-toroidal-type continuously variabletransmission unit 20 as an example of a toroidal-type continuouslyvariable transmission unit. FIG. 2 is a sectional view of a power rollerbearing 11 that functions as a thrust bearing of the toroidal-typecontinuously variable transmission unit 20.

[0040] As shown in FIG. 1, the single-cavity half-toroidal-typecontinuously variable transmission unit 20 that is used in thetransmission 21 comprises an input shaft 1 capable of being rotated bymeans of a drive source E that includes an engine or the like, an inputdisc 2, an output disc 3, a power roller 10, and a loading cam mechanism6 for use as push means.

[0041] The input disc 2 is supported on the input shaft 1 and canrotates in association with the input shaft 1. The output disc 3 isrotatably supported on the input shaft 1 so as to face the input disc 2.The output disc 3 can rotates integrally with an output transmittingmember 3 a for fetching power that is based on the rotation of the inputshaft 1. This output transmitting member 3 a can rotate in associationwith an output shaft (not shown).

[0042] The power roller 10 is provided for swinging motion between theinput disc 2 and the output disc 3, and is in rolling contact with boththe discs 2 and 3. The loading cam mechanism 6 is located on thebackside of the input disc 2.

[0043] The loading cam mechanism 6 is provided with a cam disc 4 and acam roller 5. The cam disc 4 rotates integrally with the input shaft 1in a manner such that it is fitted on a spline portion 1 a that isformed on the input shaft 1. A cam face 22 that is formed of acircumferential recess and projection are formed on that surface of thecam disc 4 which faces the input disc 2. The cam disc 4 can rotates inassociation with the drive source E.

[0044] The cam roller 5 is located between the cam disc 4 and the inputdisc 2. The cam roller 5 is provided for rotation around an axis Q inthe radial direction with respect to an axis P of the input shaft 1. Aplurality of cam rollers 5 are arranged around the axis P of the inputshaft 1.

[0045] When the cam disc 4 is rotated by means of the drive source Ethat includes the engine, in the loading cam mechanism 6 with theconfiguration described above, the cam rollers 5 is pushed toward theinput disc 2 by means of the cam face 22. Then, the input disc 2 ispushed toward the output disc 3 by means of the cam rollers 5. A pushforce generating mechanism such as a hydraulic piston may be used inplace of the loading cam mechanism 6 that functions as the push means.

[0046] A trunnion 8 is provided between the input disc 2 and the outputdisc 3. The trunnion 8 can swing around a pivot 7 in the directionindicated by arrow R in FIG. 1. A displacement shaft 9 is provided inthe central portion of the trunnion 8. The power roller 10 is rotatablysupported on the displacement shaft 9.

[0047] The power roller 10 includes a traction portion 10 a that is inrolling contact with the input disc 2 and the output disc 3. Between theinput disc 2 and output disc 3, the power roller 10 changes its angle ofinclination in accordance with the gear ratio of the toroidal-typecontinuously variable transmission unit 20. An end portion 10 b of thepower roller 10 functions as an inner race of the power roller bearing11, which will be mentioned later.

[0048] The power roller bearing 11 for use as a thrust bearing isprovided between the trunnion 8 and the power roller 10. As shown inFIG. 2, the power roller bearing 11 is provided with the end portion 10b of the power roller 10 that serves as the inner race, an outer race13, a plurality of balls 12 as rolling elements, and a retainer 14.

[0049] As shown in FIG. 3A, the outer race 13 has an annular shape. Theouter race 13 is supported on the trunnion 8. The balls 12 have aspherical shape each. The balls 12 are arranged individually for rollingmotion between the power roller 10 and the outer race 13. The retainer14, which has an annular shape, is located between the outer race 13 andthe power roller 10. The retainer 14 holds the balls 12 for rollingmotion.

[0050] The power roller bearing 11 with the aforementioned configurationis provided for swinging motion between the input disc 2 and the outputdisc 3. When the input disc 2 is pushed toward the output disc 3 bymeans of the loading cam mechanism 6, the turning force of the inputdisc 2 that is rotated by means of the drive source E is transmitted tothe output disc 3 through the power roller 10. As the outputtransmitting member 3 a rotates integrally with the output disc 3, therotation of the input disc 2 is fetched as power.

[0051] Since the loading cam mechanism 6 pushes the input disc 2 towardthe output disc 3 in the aforesaid manner so that the power roller 10comes into rolling contact with both the input and output discs 2 and 3,load in the thrusting direction acts on the power roller 10. The powerroller bearing 11 supports the load in the thrusting direction the inputdisc 2 and the output disc 3 apply to the power roller 10, and allowsthe power roller 10 to rotate.

[0052] As shown in FIGS. 3A and 3B, a raceway groove 15 is formed on anend face 17 of the outer race 13. As shown in FIGS. 4A and 4B, a racewaygroove 16 is formed on an end face 18 of the power roller 10 in aposition opposite to the raceway groove 15 of the outer race 13. Theraceway grooves 15 and 16 are formed like rings on their correspondingend faces 17 and 18. As shown in FIGS. 3B and 4B, the raceway grooves 15and 16 have an arcuate cross section each.

[0053] Base materials that individually constitute the outer race 13 andthe power roller 10 have super-finished surfaces 23 a and 24 a that areformed by super-finishing the respective surfaces of the raceway grooves15 and 16. The surface roughness of these super-finished surfaces 23 aand 24 a is adjusted to 0.05 Ra or less. Low-friction layers 23 b and 24b are formed on the super-finished surfaces 23 a and 24 a, respectively.The super-finished surfaces 23 a and 24 a and the low-friction layers 23b and 24 b constitute treated-surface layers 23 and 24. As shown in FIG.5, a treated-surface layer 25 including a low-friction layer 25 b isformed also on the surface of each ball 12. These low-friction layers 23b, 24 b and 25 b are formed individually of lubricative substancesselected from at least one of materials including gold, silver, lead,molybdenum disulfide (MOS₂), tungsten disulfide (WS₂), andfluoroplastics. In the low-friction layers 23 b, 24 b and 25 b that areformed of lubricative substances including gold, silver, lead, andfluoroplastics, among those substances, their lubricative substances aregradually scraped from the respective surfaces of the base materials asthe power roller bearing 11 rotates. As the particles of the scrapedlubricative substances are transferred to a fellow surface that istouched by the balls 12, thin films of the lubricative substances areformed also on the fellow surface. The films of these lubricativesubstances serve to lubricate the power roller bearing 11.

[0054] Since films of molybdenum disulfide and tungsten disulfide havetheir own lubricative surfaces, these lubricative surfaces wear littleby little as they develop lubricating properties. The films ofmolybdenum disulfide and tungsten disulfide are formed on the respectivesurfaces of the balls 12 and the raceway grooves 15 and 16 bysputtering.

[0055] According to the arrangement described above, the super-finishedsurfaces 23 a and 24 a with the surface roughness of 0.05 Ra or less areformed on the respective raceway grooves 15 and 16 of the outer race 13and the power roller 10, and the low-friction layers 23 b and 24 b ofmolybdenum disulfide or the like are further formed on thesuper-finished surfaces. The low-friction layer 25 b is formed also onthe surface of each ball 12. Accordingly, the value of heat developed asthe balls 12 roll in the raceway grooves 15 and 16 can be lowered. Thus,the respective surfaces of the raceway grooves 15 and 16 are not liableto flaking, so that the life of the power roller bearing 11 of thetoroidal-type continuously variable transmission unit 20 can belengthened.

[0056] In the embodiment described above, the low-friction layers 23 band 24 b are formed on both the raceway groove 15 of the outer race 13and the raceway groove 16 of the power roller 10. Alternatively,however, the low-friction layer 23 b or 24 b may be formed on one of theraceway grooves 15 and 16. In this case, the life of the power rollerbearing 11 can be also lengthened. Thus, lowering of the lifeperformance of the power roller bearing 11 of the toroidal-typecontinuously variable transmission unit 20 can be restrained by formingthe low-friction layer 23 b or 24 b on at least one of the respectiveraceway grooves 15 and 16 of the outer race 13 and the power roller 10.

[0057] According to the present invention, the dynamic torque of thepower roller bearing 11 is lessened, and the general power transmissionefficiency of the toroidal-type continuously variable transmission unit20 is improved. If the transmission unit is applied to an automobile,therefore, the fuel consumption ratio can be improved at the same time.

Second Embodiment

[0058] A toroidal-type continuously variable transmission unit of thisembodiment, like the embodiment shown in FIG. 1, comprises an input disc2, an output disc 3, a loading cam mechanism 6, a power roller 10, apower roller bearing 11, etc. The power roller bearing 11 is composed ofan end portion 10 b of the power roller 10 that functions as an innerrace, a plurality of balls 12, an outer race 13, etc.

[0059] The respective surface regions of a raceway groove 15 of theouter race 13 and a raceway groove 16 of the power roller 10 are formedhaving treated-surface layers 23 and 24 that include residualcompression stress layers 23 c and 24 c (schematically shown in FIG. 7),respectively. The treated-surface layers 23 and 24 that include theresidual compression stress layers 23 c and 24 c are formed byshot-peening the surfaces of the raceway grooves 15 and 16,respectively. These treated-surface layers 23 and 24 that include theresidual compression stress layers 23 c and 24 c individually haveresidual compression stresses within the range from −20 kgf/mm² to −100kgf/mm².

[0060] The outer race 13 and the power roller 10 with the aforementionedconfigurations are obtained according to the following processes. First,in Step S1 in FIG. 6, a workpiece as a material is forged. After theworkpiece is formed into the shape of a nearly finished product by thisforging work, the process advances to Step S2. After the workpiece iscut in Step S2, the process advances to Step S3. After the workpiece issubjected to heat treatment such as carburizing treatment orcarburizing-nitriding treatment in Step S3, the process advances to StepS4.

[0061] After the workpiece is ground into the same shape as a product inStep S4, the process advances to Step S5. After the respective surfacesof the raceway grooves 15 and 16 and the like are subjected tosuper-finishing work and the like in Step S5, the process advances toStep S6. In Step S6, the respective surfaces of the raceway grooves 15and 16 and the like are shot-peened to obtain the outer race 13 and thepower roller 10 that have desired shapes and the residual compressionstress layers 23 c and 24 c.

[0062] As is schematically shown in FIG. 7, depressions 45 andprotuberances 46 are formed on the respective surfaces of the racewaygrooves 15 and 16, that is, the respective surfaces of the residualcompression stress layers 23 c and 24 c, by shot-peening. Since alubricant such as traction oil or the lubricative substances describedin connection with the first embodiment are held in the depressions 45,the power roller bearing 11 can be prevented from being lubricatedunsatisfactorily. Thus, the life of the power roller bearing 11 can belengthened.

[0063] In the present embodiment, shot-peening is carried out in Step S6after the raceway grooves 15 and 16 are ground in Step S4. Accordingly,the outer race 13 and the power roller 10 can be completed withoutremoving the residual compression stress layers 23 c and 24 c from therespective surfaces of the raceway grooves 15 and 16 of the outer race13 and the power roller 10. The residual compression stress layers 23 cand 24 c serve to ease stress that is generated as the balls 12 roll inthe raceway grooves 15 and 16. Thus, the fatigue life of the powerroller bearing 11 can be improved.

Third Embodiment

[0064]FIG. 8 shows processes according to a third embodiment of thepresent invention that are used to form an outer race 13 and a powerroller 10. In the processes shown in FIG. 8, the respective surfaces ofraceway grooves 15 and 16 are shot-peened in Step ST5 after cutting workis carried out in Step S4. Thereafter, the respective surfaces of theraceway grooves 15 and 16 are super-finished in Step ST6. In FIG. 8,like symbols are used to designate like processes of FIG. 6, and adescription of those processes is omitted.

[0065] If the outer race 13 and the power roller 10 are formed in theprocesses shown in FIG. 8, residual compression stress layers 23 c and24 c and super-finished surfaces 23 a and 24 a are formed on therespective surfaces of the raceway grooves 15 and 16 as is schematicallyshown in FIG. 9. More specifically, the protuberances 46 shown in FIG. 7are removed by super-finishing work (Step ST6), whereupon thedepressions 45 remain on the respective surfaces of the residualcompression stress layers 23 c and 24 c. Since the aforesaid lubricantis held in these depressions 45, the power roller bearing 11 can belubricated satisfactorily. In this case, therefore, the life of thepower roller bearing 11 can be lengthened also.

[0066] In order to ascertain the function of the power roller bearing 11according to the present invention, the inventors hereof conducted anendurance test on the toroidal-type continuously variable transmissionunit 20 that is provided with the outer race 13 and the power roller 10described above. TABLE 1 shows results of this test. TABLE 1 Life DefectInvention 100 hours None product A or more Invention 100 hours Noneproduct B or more Invention 100 hours None product C or more Comparative 65 hours Separation of raceway example A groove surfaces Comparative 68 hours Separation of raceway example B groove surfaces Comparative 59 hours Separation of raceway example C groove surfaces

[0067] In the test described above, the rotational frequency of theinput shaft 1 was set at 4,000 rpm, and input torque form the drivesource E at 390 Nm. Further, the endurance test was conducted with useof synthetic traction oil as lubricating oil and with the temperature ofthis lubricating oil at 100° C.

[0068] In Table 1, a product A of the present invention and a product Bof the present invention uses outer races and power rollers that wereformed individually in the processes shown in FIG. 8. A product C of thepresent invention uses a outer race and a power roller that were formedin the processes shown in FIG. 6. Comparative examples A, B and C useouter races and power rollers in which the respective surfaces of theirraceway grooves were not shot-peened.

[0069] According to the test results shown in Table 1, none of theshot-peened products A, B and C of the present invention was broken in atest time of 100 hours. In all of the comparative examples A, B and C,the respective surfaces of the raceway grooves underwent flaking in atest time of about 60 hours.

[0070] Thus, power roller bearings 11 of high life performance were ableto be obtained in a manner such that the respective surfaces of theraceway grooves 15 and 16 were shot-peened to form the treated-surfacelayers 23 and 24 including the residual compression stress layers 23 cand 24 c.

[0071] In order to apply a relatively high residual compression stressof, e.g., −100 kgf/mm² to the residual compression stress layers 23 cand 24 c, shots must be struck hard against the respective surfaces ofthe raceway grooves 15 and 16. In this case, fine cracks sometimes maybe formed in the surfaces of the raceway grooves 15 and 16. These finecracks can be removed by effecting super-finishing work (Step ST6) aftercarrying out shot-peening operation (Step ST5) in the processes shown inFIG. 8.

[0072] In the second and third embodiments described above, the residualcompression stress layers 23 c and 24 c are formed on both therespective raceway grooves 15 and 16 of the outer race 13 and the powerroller 10. Alternatively, however, the residual compression stress layer23 c or 24 c may be formed on one of the raceway grooves 15 and 16. Inthis case, the life of the power roller bearing 11 can be lengthenedalso. Further, the low-friction layer 25 b described in connection withthe first embodiment may be formed on the surface of each ball 12. Inthis case, the residual compression stress layers 23 c and 24 c isformed on the raceway grooves 15 and 16 by shot-peening. Alternatively,the low-friction layer 23 b or 24 b may be formed on one of the racewaygrooves 15 and 16. In this case, the residual compression stress layer23 c or 24 c is formed on the other by shot-peening. Thus, the life ofthe power roller bearing 11 of the toroidal-type continuously variabletransmission unit 20 can be lengthened by forming the treated-surfacelayers 23 and 24 on at least one of the respective surfaces of theraceway groove 15 of the outer race 13 and the raceway groove 16 of thepower roller 10.

[0073] It is to be understood, moreover, that any other portions thanthe raceway grooves 15 and 16 may be shot-peened. The aforesaid residualcompression stress layers 23 c and 24 c may be formed by barreling orthe like instead of shot-peening.

[0074] The improvement of the rolling fatigue life by the aforesaidshot-peening is also applicable to individual traction surfaces of thetoroidal-type continuously variable transmission unit 20. Thus,conditions for these traction surfaces, including high temperature, highspeed, high contact pressure, presence of spins, and use of tractionoil, etc., are similar to those for the power roller bearing 11. Thetechnique of the present invention is also effective for the tractionsurfaces. The results of the experiment shown in Table 1 indicate thatthis technique is also effective for the traction surfaces. Morespecifically, the life of the discs 2 and 3 or the power roller 10 canbe lengthened by shot-peening at least one of the respective tractionsurfaces of the discs and the power roller.

[0075] As is evident from the above description, the power rollerbearing of the present invention can be suitably used for a transmissionof, for example, a general industrial machine or transportationequipment, as well as a power transmission system of an automobile. Thepresent invention is applicable to full-toroidal-type continuouslyvariable transmission units as well as to half-toroidal-typecontinuously variable transmission units.

[0076] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details and representativeembodiments shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. In a power roller bearing of a toroidal-typecontinuously variable transmission unit, which is provided for swingingmotion between an input disc capable of being rotated by means of adrive source and an output disc opposed to said input disc, the powerroller bearing of the toroidal-type continuously variable transmissionunit comprising: an outer race; a power roller in rolling contact withsaid input disc and output disc; and balls provided for rolling motionbetween said outer race and said power roller, said outer race and saidpower roller having annular raceway grooves in which said balls rollindividually, at least one of the respective raceway grooves of saidouter race, heat-treated and ground, and said power roller being formedhaving treated-surface layers including super-finished surfaces with thesurface roughness of 0.05 Ra or less and residual compression stresslayers shot-peened so that the residual compression stress thereinranges from −20 kgf/mm² to −100 kgf/mm².
 2. In a power roller bearing ofa toroidal-type continuously variable transmission unit, which isprovided for swinging motion between an input disc capable of beingrotated by means of a drive source and an output disc opposed to saidinput disc, the power roller bearing of the toroidal-type continuouslyvariable transmission unit comprising: an outer race; a power roller inrolling contact with said input disc and output disc; and balls providedfor rolling motion between said outer race and said power roller, saidouter race and said power roller having annular raceway grooves in whichsaid balls roll individually, both of the respective raceway grooves ofsaid outer race and said power roller being formed having super-finishedsurfaces with the surface roughness of 0.05 Ra or less and low-frictionlayers on at least one of the super-finished surfaces, the low-frictionlayers being formed of at least one of lubricative substances includinggold, silver, lead, molybdenum disulfide, tungsten disulfide, andfluoroplastics.